Metformin, a widely prescribed anti-diabetic drug, has recently drawn much attention due to its intriguing anti- neoplastic effects. However, despite the increasing body of evidence supporting metformin's effectiveness in cancer prevention and therapy, the underlying mechanisms remain poorly understood. The proteotoxic stress response, mediated by Heat shock factor 1 (HSF1), is an evolutionarily conserved adaptive mechanism that enhances cellular and organismal survival in face of a wide variety of stressors from without and within. In sharp contrast to this well-recognized beneficial effect, emerging studies have begun to reveal a surprising role of this stress response in facilitating oncogenesis. Based on our preliminary results, we hypothesize that metformin exerts its anti-cancer effects in part through suppressing the HSF1-mediated proteotoxic stress response and subsequently disrupting cancer proteostasis. The objectives of this proposal are to define the molecular mechanisms by which metformin suppresses HSF1 and to determine whether this suppression contributes to metformin's anti-cancer effects. Two specific aims will be pursued: Aim 1) Determine whether metformin suppresses HSF1 through activation of the key metabolic sensor AMPK signaling by biochemical and genetic approaches, the molecular mechanisms by which metformin regulates HSF1 will be interrogated; Aim 2) Determine whether metformin inactivates HSF1 and consequently disrupts proteostasis, thereby exerting its anti-cancer effects. This will be tested in a genetic melanoma xenograft model. Successful completion of this project will elucidate a new molecular mechanism of action by which metformin protects against cancer incidence and mortality, which would be particularly instructive to the numerous ongoing clinical trials of metformin. More importantly, in light of the emerging role of HSF1 in a broad spectrum of human cancers, success of this project will authenticate targeting the HSF1-mediated stress response as a promising anti-cancer strategy in general.